A new free-living marine nematode species, Oncholaimus moanae sp. nov., is described from intertidal fine sand in southern New Zealand. Oncholaimus moanae sp. nov. can be distinguished from other species of the genus by the presence of a pre-cloacal papilla bearing four pairs of short, stout spines, a post-cloacal papilla, long (>70 μm) spicules, and a demanian system with two openings situated laterally at level of uvette. The δ13C signature of O. moanae sp. nov. suggests that benthic microalgae are the main carbon source for this species, but an elevated δ15N signature suggests predatory feeding habits. The fatty acid composition of O. moanae sp. nov. is rich in highly unsaturated fatty acids, which are likely to originate from heterotrophic protists (e.g. ciliates). The data obtained in this study suggest, for the first time, that marine nematodes can be a high quality food source (i.e. rich in highly unsaturated fatty acids) to predators. Large nematodes living near or at the sediment surface, in particular, may represent an important trophic link between heterotrophic protists and higher trophic levels in marine sediments.